Scintillators are materials that absorb high-energy radiation (e.g. gamma rays, x-rays, high-energy particles) and emit light (low-energy photons) in response. To the extent that the number of emitted photons is proportional to the total energy of the stopped radiation, a scintillator provides the useful function of identifying the total energy of any radiation that is stopped.
As the nucleus of every element (and each isotope of each element) emits a characteristic fingerprint of gamma-ray energies when suitably excited (usually by neutron activation), scintillator responses to such emissions provide value in security screening of shipping and trucking containers and baggage, where chemical elements and isotopes of elements can be identified (and imaged using arrays of segmented scintillators) without opening the container. This applies both to radioactive elements (nuclear nonproliferation and screening) and to ordinary non-radioactive elements via neutron activation of gamma emission. Scintillators are also widely used in medical imaging and diagnostics as well as oil-well logging, where energy resolution can often also be of value. Desirable energy resolution requires sufficient proportionality of light yield to ray energy. Non-proportionality of scintillator photonic emission to the stopped radiation can degrade resolution and produce significant inaccuracies in determining the total energy of the radiation received.
It is generally accepted that non-proportionality in scintillators is associated with quenching (non-radiative electron-hole recombination) in parts of the particle/ray track wherein ionization density is high, coupled with the characteristic variability of dE/dx from beginning to end of an electron track. Prior scintillators, such as CsI:Tl as well as others, comprise crystalline materials doped with impurities. The impurities serve as radiative recombination centers for electron-hole pairs generated from the absorption of the high energy radiation. Impurities are doped throughout the host crystal to ensure efficient photonic output from the scintillator upon radiation absorption. As a result, an electron-hole pair does not have to travel far before contacting an impurity for radiative recombination. This limitation of carrier mobilities reduces the probability of linear (i.e. trap dominated) non-radiative electron-hole recombination, thereby maximizing the light output of the scintillator. Maximization of light output by this route, however, has associated costs as the limitation of carrier mobility by high dopant levels throughout the host crystal can result in or exacerbate non-proportional response of the scintillator to the absorbed radiation.